The goal was to reduce common setup of messages. Changes:
* MailBox turned into singleton to follow existing patterns
* Removed device specific messages from MailBox requiring
clients to know the details instead
* Created base Message class which clients should deriver from
It really simplify the usage for more complicated message queues
like framebuffer setup - see followup commits.
When booting on RPI3 firmware puts CPU in EL2 mode which is
different from QEMU's default EL3.
I've added logic to discover initial mode at boot
and then act accordingly. This results in Serenity corectly
switching to EL1 on target hardware now.
For now, this can only query microseconds since boot.
Use this to print a timestamp every second. This busy-loops
until a second has passed. This might be a good first use of
interrupts soon.
qemu used to not implement this timer at some point, but
it seems to work fine even in qemu now (qemu v 5.2.0).
As a demo, query the firmware version. `Meta/serenity.sh gdb aarch64`
can be used to observe that qemu puts 0x548E1 in x0 in response
to this mailbox message.
Needed for functions that have local variables.
In time we need to share this between aarch64 and intel, but while
we figure out what exactly the aarch64 Prekernel should do, let's
duplicate this.
